Make your simple syrup by boiling half a cup of water and mixing in half a cup of sugar. Stir until fully dissolved and cool completely. With a muddler or the bottom of a wooden spoon, tamp the tomatoes and three mint leaves a few times in the bottom of a cocktail shaker. Add the rest of the ingredients. Fill the shaker to the brim with ice, seal, and shake hard. Strain into a chilled cocktail glass and garnish with a small mint leaf.
Notes: The flavor of heirloom tomatoes vary; adjust the lemon and simple syrup according to the tomatoes on hand, and your own taste.